The present invention relates generally to casting of metals, and more specifically, relates to a mold basket and to an improved ceramic mold used in a metal bath furnace for the directed solidification of superalloys.
Certain components, such as for example turbine blades and stator vanes for gas turbine engines, because of their relatively complex shapes and harsh operating environment to which these components are exposed, are typically cast of nickel-based and cobalt-based alloys which are conventionally known as superalloys, which have high strength and typically very high melting temperatures.
The strength of such components is enhanced by forming the turbine vanes (stators) and particularly the turbine blades using directional solidification casting for obtaining substantially single crystal components. Such process is conventionally known.
Various processes and apparatus for directional solidification casting, such as directional solidification casting apparatus and method disclosed in U.S. Pat. Nos. 4,108,236 and 4,175,609, are known in the industry and vary in effectiveness. In these processes, a suitable ceramic mold is specifically configured for the particular component being cast, such as a gas turbine engine blade or vane. The mold is lowered into a heating chamber where it is pre-heated, and subsequently then filled with a desired superalloy in a superheated liquid melt condition. Thereafter, the bottom of the mold is then subjected to preferential cooling to commence the unidirectional solidification process necessary for single crystal formation, which travels upwardly through the mold.
Cooling of the mold may be accomplished in different manners. In one conventional process, a suitable liquid metal coolant, such as molten tin or aluminum, is contained in a bath below the mold, with the mold then being immersed into the cooling bath for effecting a substantially large temperature gradient in the melt for enhancing directional solidification.
In a typical directional solidification casting furnace, solid superalloy known as a xe2x80x9cchargexe2x80x9d, is initially placed inside a melting crucible surrounded by a suitable heater, such as an induction heater, which melts the charge to form the liquid melt with suitable superheat. In this respect, the mold is initially positioned inside a heating chamber within a furnace, which preheats the mold to a suitable elevated temperature. These components are typically disposed within a common pressure vessel or housing which makes up the furnace, which is typically evacuated, or filled with a suitable inert gas.
During the process, the melt is poured from the melting crucible into preheated mold. The mold is then lowered, bottom end first, into the bath for immersion cooling thereof to directionally solidify the melt upwardly inside the mold. Upon completion of melt solidification inside the mold, the mold is removed upwardly from the bath, furnace, and housing. A new charge and mold are placed inside the housing and the process is repeated to cast additional parts.
In lowering the mold into the liquid metal bath, the mold is supported within an elevator chamber which may be moved up and down by a piston or equivalent motive means. In this respect, the mold, conventionally, rests on a chill plate of a basket-like structure which extends downwardly within the elevator chamber. The chill plate is adapted to effect cooling of the mold by conducting heat from the mold to the liquid metal bath.
One such mold basket is particularly described in EPO Serial No. 0631832A1 filed Feb. 2, 1993. As can be seen from FIGS. 1, 2 and 3 of EPO Serial No. 0631832A1, the mold basket disclosed therein offers no lateral support for the mold. The mold is, therefore, relatively dimensionally unstable, such condition potentially leading to spillage of the alloy or even a fracture of the mold. Further, by virtue of being fixedly secured to the elevator, the mold basket in EPO Serial No. 0631832A1 has no means of dissipating the thermal stresses realized during heating and cooling processes within the furnace and metal bath. This may eventually cause fracture of the mold basket, leading to substantial maintenance work for clean-up and consequent downtime.
The shape of the mold is configured to the shape of the article being cast. In some cases, by virtue of such shape, the mold is not dimensionally stable, especially when only supported from its bottom by the above-described chill plate. As a result, reliability of the casting process is compromised. Additional matter could be added to the mold to increase its weight and, therefore, enhance its stability. However, this impedes heat transfer, hence compromising the solidification process.
The present invention discloses a mold basket, for supporting a mold, for use in a liquid metal bath furnace with an elevator.
In one of its broad aspects, the present invention provides a mold basket, for supporting a mold, for use in a liquid metal bath furnace with an elevator, comprising a flange for suspending the mold basket from the elevator; and a horizontal plate disposed beneath the flange for supporting the mold, wherein the plate is coupled to the flange with a plurality of vertical tie rods. The flange includes an aperture for receiving the mold, wherein the aperture is defined by an inner wall of the flange and the mold is substantially laterally surrounded about its upper edge by the inner wall so as to be laterally supported thereby.
In another aspect, the present invention provides a mold basket, for supporting a mold, for use in a liquid metal bath furnace with an elevator, comprising a basket flange for suspending the mold basket from the elevator; a plate vertically disposed beneath the flange for supporting the mold; and a plurality of tie rods for coupling the flange to the plate, each of the tie rods having first and second ends, wherein the first end is received in a corresponding rod aperture in the flange and the second end is received in a corresponding rod aperture in the cooling plate. Preferably, the rod apertures are guide slots. Furthermore, the mold basket wherein each of the first and second ends of the tie rods are comprised of first and second rod flanges respectively, wherein the first rod flange engages a top surface of the basket flange and the second rod flange engages a bottom surface of the cooling plate for effecting suspension of the cooling plate from the basket flange.
In yet another aspect, the present invention provides a combination mold and mold basket, the mold basket for supporting the mold during cooling thereof in a liquid metal bath furnace having an elevator means for raising and lowering the mold, the mold basket comprising a flange for suspending the mold basket from the elevator means and a horizontal plate vertically disposed beneath the flange for supporting the mold, the plate being coupled to the flange with a plurality of tie rods, wherein the mold has a nesting device for mating engagement with the plate.
The present invention also provides an apparatus for casting directional solidified metal articles comprising: a furnace with a heating chamber for preheating a mold, a melting crucible surrounded by a suitable heater for heating and melting metal, pouring means to pour the heated metal into the preheated mold, a crucible member disposed beneath the heating chamber and containing a liquid metal bath, an elevator for lowering the mold into the liquid metal bath, and a mold basket for supporting the mold, wherein the mold basket comprises a flange for suspending the mold basket from the elevator, and a cooling plate vertically disposed beneath the flange for supporting the mold, and wherein the cooling plate is coupled to the flange with a plurality of tie rods.